1. Field of the Invention
The present invention relates to a charging material made from semiconductor material, for charging or recharging a melting crucible during the Czochralski crucible-pulling process. The invention also relates to a holding system for the charging material. The invention furthermore relates to a holding system between two semiconductor rods.
2. The Prior Art
During the production of single crystals using the Czochralski crucible-pulling process, silicon fragments or silicon granules are introduced into a crucible and are melted. Since the specific bulk density of the fragments or granules is lower than the theoretical specific density of silicon, the filling level of the crucible falls during the melting. In order, however, to obtain a high yield, polycrystalline silicon granules or polycrystalline silicon rods are added to the molten silicon an are also melted.
The Czochralski crucible-pulling process is extensively explained in, for example, W. Zulehner and D. Huber, Czochralski-Grown Silicon, Crystals 8, Springer Verlag, Berlin-Heidelberg, 1982, and the literature cited therein, taking particular account of what is currently the most important application area, namely the crucible pulling of silicon single crystals. In many cases, there has been a move toward improving the filling level of the crucible by adding further solid melting material in the form of semiconductor rods. These rods are added after the initial melting, to the melt which has been formed from the lumpy material which was initially charged. To do this, polycrystalline pieces of rod are dipped, as charging material or recharging material, into the exposed surface of the melt. This is done by means of suitable holders or holding systems, generally before the actual pulling operation commences. These pieces are gradually melted until the desired melt level is reached.
During production of single crystals using the float zone process, polycrystalline silicon rods are melted on a zonal basis by means of a high-frequency coil (H. Hadamovsky, Werkstoffe der Hableitertechnik, Chapter 6, VEB-Verlag, Leipzig, 1985).
A common feature of both processes is that-the polycrystalline silicon rods have to be held on rotating shafts in the pulling installations.
U.S. Pat. No. 5,888,293 has disclosed a recharging material and a holding system for the recharging material. The recharging material comprises a multiplicity of polycrystalline silicon rods which are linked by means of connecting pieces. The holding system comprises an encircling flute which is arranged on the outside of the lateral surface of the polycrystalline silicon rod and a gripper system made from various materials, such as for example from quartz, tantalum, or molybdenum. The drawback of this holding system is that the high-purity molten silicon is contaminated by the materials used in the gripper system.
The drawback of the charging material is that it is not possible to charge the semiconductor material and pull a single crystal from the melt in a single step. This is because first of all the charging material has to be melted and only then can a seed crystal be brought,to the surface of the melt. However, it is desirable for the charging and pulling of a single crystal to be carried out in a single step. To do this, charging material and monocrystalline seed crystal have to be connected to one another. Accordingly, with this technique it is not possible subsequently to use a monocrystalline rod holder as a seed.
Various methods of holding polycrystalline rods in pulling installations have been disclosed in the prior art. By way of example, holding systems which provide connection by means of a weld between a monocrystalline holding rod and the polycrystalline silicon rod are known from DE 39 22 135. This connecting technique is highly complex. The polycrystalline. silicon rod must first be provided with a flute and a cone. Then, the polycrystalline silicon rod is fitted at the flute, for example in a float zone pulling installation. Then, at the other end, i.e. at the cone, the monocrystalline rod holder is welded to the polycrystalline silicon rod. Only then is it possible to use a holding arrangement of this type. The partially fused monocrystalline rod holder can only be reused after extensive re-machining.
Therefore an object of the present invention is to provide a holding system, in particular for charging material, which is distinguished by a high strength and which can be produced without complex thermal treatment of the semiconductor rods and without using other materials.
Another object of the invention is to provide a charging material which avoids the drawbacks of the prior art and is suitable for charging and pulling a single crystal from the melt.
The above objects are achieved by a charging material made from semiconductor material, for charging or recharging a melting crucible during the Czochralski crucible-pulling process, which has a polycrystalline semiconductor rod, which at one end is designed as a groove, and a monocrystalline semiconductor rod, which at one end is designed as a tongue, which rods are coupled by means of a tongue-and-groove connection.
Surprisingly, it has been found that it is possible to produce a tongue-and-groove connection between hard, brittle materials, such as for example a polycrystalline semiconductor rod and a monocrystalline semiconductor rod, preferably a highs strength monocrystalline seed crystal. Another surprise was that this connection is able to withstand bending stresses which are caused by suspending the monocrystalline semiconductor rod in a non-coaxial manner with respect to the rod axis, although the seed crystal is small and at risk of breaking compared to the semiconductor rod.
According to the invention, complex process steps, such as fusion of the seed, the use of foreign materials in the holder or the holding system and therefore contamination of the product are avoided by the holding system. Furthermore, fitting the rods into the pulling installations is simplified and it is possible for the polycrystalline semiconductor rods and the monocrystalline semiconductor rods, in particular the monocrystalline seed crystal, to be transported separately from one another. Fundamentally, the holding system can be reused and the connection point is of high strength.
Accordingly, the invention also relates to a holding system which makes it possible to secure polycrystalline silicon rods in all float zone and crucible-pulling processes without complex heat treatment of the connection point and without foreign materials at the connection point. The holding system according to the invention has a tongue-and-groove connection between a pollycrystalline semiconductor rod, which at one end is designed as a groove, and a monocrystalline semiconductor rod, for example a seed crystal, which at one end is designed as a tongue.
The holding system is preferably used to hold pollycrystalline silicon rods which are added to the molten silicon during the Czochralski crucible-pulling process. The holding system is preferably used to hold polycrystalline silicon rods which are melted on a zonal basis during the float zone process.
The holding system is a releasable mechanical connection between a polycrystalline semiconductor rod, in particular a silicon rod, and a monocrystalline semiconductor rod, in particular a silicon rod, preferably a monocrystalline silicon seed crystal, which can be obtained without using additional tools or materials.
The holding system is distinguished by complete reusability, if appropriate after the holding rod made from monocrystalline silicon has been cleaned, in processes in which the monocrystalline semiconductor rod is not used as a seed crystal.
Another advantage of the holding system according to the invention is a separately transportable holder made from hyperpure silicon, in particular from monocrystalline hyperpure silicon, which only has to be fitted when the polycrystalline semiconductor rod is being introduced into float zone or crucible-pulling apparatus.
A further advantage is that production of the groove can be readily automated; allowing a multiplicity of polysilicon rods to be machined in a single machining step.